test(migrations)+fix: widen drift check to column shape; notifications.created_at NOT NULL (S2 review #2)

The drift check dropped every diff except table/column presence, so a column
type or nullability change would slip through. Add modify_type/modify_nullable
to the ops it fails on. That immediately surfaced a real pre-existing drift:
notifications.created_at was created NULLABLE by migration 0017 but the model
has always declared it NOT NULL (Mapped[datetime] + server_default=func.now()).
Migration 0066 enforces NOT NULL to match the model — safe, since the
server_default has filled every row since day one.

backend/alembic/versions/0066_notif_created_at_notnull.py

@@ -0,0 +1,24 @@
+"""notifications.created_at NOT NULL (align schema to the model)
+
+The model has always declared `Notification.created_at: Mapped[datetime]` (NOT NULL) with
+`server_default=func.now()`, but migration 0017 created the column without `nullable=False`, so the live
+schema allowed NULL — a model/schema drift the new migration drift-test (E1 S2) surfaced. The
+server_default has filled every row since day one, so no NULLs exist; enforcing NOT NULL just makes the
+schema match the contract the model already promises. Pure DDL, reversible.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0066_notif_created_at_notnull"
+down_revision: Union[str, None] = "0065_plex_detail_fields"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.alter_column("notifications", "created_at", nullable=False)
+
+
+def downgrade() -> None:
+    op.alter_column("notifications", "created_at", nullable=True)